1. Field of the Invention
This invention pertains generally to systems for monitoring railroad vehicles and, more particularly, to such systems for determining the position of a train. The invention also pertains to methods for determining the position of a railroad vehicle.
2. Background Information
In the art of railway signaling, traffic flow through signaled territory is typically directed by various signal aspects appearing on wayside indicators or cab signal units located on board railway vehicles. The vehicle operators recognize each such aspect as indicating a particular operating condition allowed at that time. Typical practice is for the aspects to indicate prevailing speed conditions.
For operation of this signaling scheme, a track is typically divided into cascaded sections known as “blocks.” These blocks, which may be generally as long as about two to about five miles, are electrically isolated from adjacent blocks by typically utilizing interposing insulated joints. When a block is unoccupied, track circuit apparatus connected at each end are able to transmit signals back and forth through the rails within the block. Such signals may be coded to contain control data enhancing the signaling operation. Track circuits operating in this manner are referred to as “coded track circuits.” One such coded track circuit is illustrated in U.S. Pat. No. 4,619,425. When a block is occupied by a railway vehicle, shunt paths are created across the rails by the vehicle wheel and axle sets. While this interrupts the flow of information between respective ends of the block, the presence of the vehicle can be positively detected.
In the case of trains in signaled territory, control commands change the aspects of signal lights, which indicate how trains should move forward (e.g., continue at speed; reduce speed; stop), and the positions of switches (normal or reverse), which determine the specific tracks the trains will run on. Sending the control commands to the field is done by an automated traffic control system, or simply control system. Control systems are employed by railroads to control the movements of trains on their individual properties or track infrastructures. Variously known as Computer-Aided Dispatching (CAD) systems, Operations Control Systems (OCS), Network Management Centers (NMC) and Central Traffic Control (CTC) systems, such systems automate the process of controlling the movements of trains traveling across a track infrastructure, whether it involves traditional fixed block control or moving block control assisted by a positive train control system. The interface between the control system and the field devices is typically through control lines that communicate with electronic controllers at the wayside, which in turn connect directly to the field devices.
In dark (unsignaled) territory, forward movement of trains is specified in terms of mileposts (e.g., a train is given the authority to move from its current location to a particular milepost along its planned route), landmarks or geographic locations. Controlling the movements of trains is effected through voice communication between a human operator monitoring the control system and the locomotive engineer. The operator is responsible for authorizing the engineer to move the train and to manually perform state-changing actions, such as throwing switches, so that the train is able to follow the operator-specified route. Typical railroad voice exchanges are prescribed conversations involving specific sequences of sentences that fit the situation. For example, the engineer will periodically report the train's position by telling the dispatcher “Train BX234 is by Milepost 121.4”. The operator will repeat the position report back to the engineer while entering it into the Computer Aided Dispatching system. The engineer will validate the entry by saying “That is correct” or some similar phrase, standard for that railroad. In this way, the operator knows where all trains are and the limits of their movement authorities so that the operator is able to direct their movements in a safe manner.
At least one alternative train positioning system (ERTMS) utilizes a system of short range radio frequency transmitter/receiver pairs. As the train approaches a protected area, such as a grade crossing or switching interchange, the onboard transmitter emits a signal that elicits a response from the wayside installation. The exchange between the system onboard the train and the wayside installation causes the train to update its position (by observed proximity to the transmitter) and be granted movement authority (delivered to the train by a wayside transmitter from a network operations center). The ERTMS system has been observed to require considerable preparation and careful installation.
Other known systems and methods determine train position. For example, U.S. Pat. No. 4,790,191 discloses a dead reckoning and map matching process in combination with Global Positioning System (GPS) sensors. When relative navigation sensors (e.g., vehicle odometer; differential odometer) are providing data within an acceptable error, the system does not use the GPS data to update the vehicle's position. The system does use GPS data to test whether the data from the relative sensors are within the acceptable error. If not, the system resets the vehicle's position to a position calculated based on the GPS data and then the system performs a “dead reckoning” cycle followed by “map matching”.
U.S. Pat. No. 5,862,511 discloses a vehicle navigation system and method that uses information from a GPS to obtain velocity vectors, which include speed and heading components, for “dead reckoning” the vehicle position from a previous position. If information from the GPS is not available, then the system uses information from an orthogonal axes accelerometer, such as two or three orthogonally positioned accelerometers, to propagate vehicle position. The system retains the accuracy of the accelerometers by repeatedly calibrating them with the velocity data obtained from the GPS information.
U.S. Pat. No. 5,948,043 discloses a navigation system for tracking an object, such as an automobile as it moves over streets, using an electronic map and a GPS receiver, and claims that the system functions without using data from navigation sensors other than one or more GPS sensors. The GPS receiver accepts data from a number of satellites and determines a GPS derived position and velocity. Based on the previous position of the object, the GPS derived position, the velocity, the dilution of precision (DOP), and the continuity of satellites for which data is received, the system determines whether the GPS data is reliable. When determining whether the GPS data is reliable, the first step is to compare the GPS derived position to the previous position (e.g., from map matching). If the GPS data is reliable, then the previous position of the object is updated to the GPS derived position. The updated position is then matched to a map of roads.
U.S. Patent Application Publication No. 2003/0236598 discloses an integrated railroad traffic control system that links each locomotive to a control center for communicating data and control signals. Using on-board computers, GPS and two-way communication hardware, rolling stock continuously communicate position, vital sign data, and other information for recording in a data base and for integration in a comprehensive computerized control system. The position of each train is determined in real time by the use of a conventional positioning system, such as GPS, and is communicated to the dispatcher, so that the progress of each train can be followed and compared to the expected schedule expressed in the relevant train graph and panel. A separate channel is used to receive, record and transmit signals from mile-mark tag readers placed along the tracks in order to periodically confirm the exact position of the train. These signals are emitted by sensors that detect and identify specific tags placed wayside while the train is passing by. Since they are based on precisely fixed markers, the train positions so recorded are used to double-check and, if necessary, correct corresponding GPS positioning data. An input/output channel is provided to receive, record and transmit data from vital sign sensors on the train, such as pressure and/or temperatures of hydraulic systems and other operating parameters deemed important for safe and efficient maintenance and operation.
U.S. Pat. No. 6,496,778 discloses three conventional approaches for integrating GPS and an inertial navigation system (INS). The first approach is to reset directly the INS with the GPS-derived position and velocity. The second approach is cascaded integration where the GPS-derived position and velocity are used as the measurements in an integration Kalman filter. The third approach is to use an extended Kalman filter which processes the GPS raw pseudorange and delta range measurements to provide optimal error estimates of navigation parameters, such as the inertial navigation system, inertial sensor errors, and the global positioning system receiver clock offset.
A Kalman filter is an efficient recursive filter that estimates the state of a dynamic system from a series of incomplete and noisy measurements. For example, in a radar application, where one is interested in tracking a target, information about the location, speed and acceleration of the target is measured with a great deal of corruption by noise at any instant of time. The Kalman filter exploits the dynamics of the target, which govern its time evolution, to remove the effects of the noise and get a good estimate of the location of the target at the present time (filtering), at a future time (prediction), or at a time in the past (interpolation or smoothing). The Kalman filter is a pure time domain filter, in which only the estimated state from the previous time step and the current measurement are needed to compute the estimate for the current state. In contrast to batch estimation techniques, no history of observations and/or estimates are required. The state of the filter is represented by two variables: (1) the estimate of the state at time k; and (2) the error covariance matrix (a measure of the estimated accuracy of the state estimate). The Kalman filter has two distinct phases: Predict and Update. The Predict phase uses the estimate from the previous time step to produce an estimate of the current state. In the Update phase, measurement information from the current time step is used to refine this prediction to arrive at a new, (hopefully) more accurate estimate.
The Kalman filter technique depends critically on a well tuned covariance matrix, which, in turn, depends critically on the dynamics of the modeled system. Train dynamics, while well understood and predicable in controlled circumstances are notoriously variable in actual operation, due largely to the variability of the loads applied. Thus, claims of vitality for position systems that rely on the Kalman filtering technique are believed to be difficult to demonstrate.
U.S. Pat. No. 6,826,478 discloses that various auxiliary input data are provided to a Kalman filter which processes the auxiliary input data to determine and provide state corrections to an inertial navigation and sensor compensation unit. These state corrections from the Kalman filter are used by the inertial navigation and sensor compensation unit to enhance the accuracy of position, velocity, attitude and accuracy outputs, thereby enhancing the accuracy of the aided inertial navigation system (AINS). The auxiliary input data includes GPS data, speed data, map information, wheel angle data, and other discrete data, such as from transponders or rail detectors if the AINS is applied to a railcar or other similar applications. The AINS calculates the distance to the next map point. This information may be desirable for various applications in modern railcars, such as positive train control, in which various functions and operations of the train are automated. Such calculated distance is based on the best estimate of position, in which case there may be sudden changes if the quality of the input data improves suddenly, again for example, if GPS data is reacquired.
U.S. Pat. No. 6,826,478 also discloses that the calculated distance along the path is always smoothly changing. An illustration depicts a confidence value as a confidence circle. A mobile object is at a determined position along the path or track. The confidence circle indicates that the actual position of the mobile object is within the confidence circle from the determined position. As the confidence circle decreases in size, the distance that the determined position can deviate from the actual position of the mobile object decreases, and vice versa.
U.S. Patent Application Publication No. 2002/0062193 discloses a geospatial database access and query method, such as a map and Inertial Measurement Unit/Global Positioning System (IMU/GPS) navigation process. This supports real time mapping by using IMU/GPS integrated system as the positioning sensor. A point query is aimed at finding the node (connected or entity) in the vicinity of the query point. The vicinity area is defined as a circle on the screen with a radius and centered at the query point. The location data from the map matching process module is fed to a Kalman filter that blends the measurements from an Inertial Measurement Unit and a GPS receiver to further correct navigation errors.
U.S. Pat. No. 6,641,090 discloses a train location system and method of determining track occupancy. The system utilizes inertial measurement inputs, including orthogonal acceleration inputs and turn rate information, in combination with wheel-mounted tachometer information and GPS/DGPS position fixes to provide processed outputs indicative of track occupancy, position, direction of travel and velocity. Various navigation solutions are combined together to provide the desired information outputs using an optimal estimator designed specifically for rail applications and subjected to motion constraints reflecting the physical motion limitations of a locomotive. A rate gyro, a first accelerometer board and a second accelerometer board provide, respectively, rate of turn and three-axis acceleration information to processing electronics. Information vectors from sources having different error characteristics are geo-reconciled to reduce the adverse effect of short- and long-term errors. In the context of the velocity vector, for example, an inertially derived velocity vector is geo-reconciled with a geo-computed velocity vector obtained, for example, from the calibrated wheel tachometer and the train forward axis or track centerline axis. In general, the inertially obtained and tachometer derived velocity vectors will be different based upon the cumulative errors in each system. An optimal estimator functions to blend two such values to obtain the geo-reconciled velocity vector. With each successive computation sequence, the optimal estimator functions to estimate the error mechanisms and effect corrections to successively propagate position and the associated uncertainty along the track. A main process module fuses three inertial navigation solutions together, aided by exogenous GPS/DGPS receiver data and tachometer data in a position computation (Kalman) optimal estimator. The three navigation solutions include: (a) conventional strapdown navigation solution using a single Z-axis gyro and nulled x- and y-channels; (b) a projection of the inertial data along the occupied track profile reconstructed from parameters on the fly, and then being integrated appropriately (e.g., for position; speed); and (c) projection of the inertial data along the locomotive (cab) fixed reference axes and then being appropriately integrated for location. The three navigation solutions are optimally blended with the external GPS/DGPS receiver and the tachometer data, and the solution is subjected to motion constraints reflecting the physical limitations of how a locomotive can move.
U.S. Patent Application Publication No. 2005/0107954 discloses a collision warning and avoidance system which includes an integrated on-board Train Navigation Unit and a GPS Interface Subsystem to locate a train. The system includes a GPS location signal, fixed transponder stations, and a calibrated, rectified transponder identification subsystem for scanning the track based transponders for override of train controls in the event of a collision risk. A database includes all transponders, their location and the track ID on which they are located. A logic associative memory is in communication with a control signal generator, which is capable of emitting a signal responsive to input data to override train controls to effect braking in the event of a collision risk.
There is room for improvement in systems and methods for determining the position of a railroad vehicle with respect to both accuracy and vitality.